PROVENANCE OF THE MUDDY CREEK FORMATION AND ITS RELATIONSHIP TO COLORADO RIVER EVOLUTION

We conducted a basin analysis of the exposed upper portion of the Miocene Muddy Creek Formation (MCF) in the Mesquite basin in Nevada and Arizona. A primary goal was to test the hypothesis that these strata were deposited by a paleo-Colorado River that flowed west through the Virgin River Gorge (VRG), prior to the inception of the Colorado River exiting the Grand Canyon via the Grand Wash trough further to the south.

Our results reveal that the MCF adjacent to the modern Virgin River coarsens upward from dominantly siltstone with some evaporates to dominantly sandstones with high velocity flow sedimentary structures (planar laminated and trough cross-bedded with subordinate mudstone) and is capped by pebble to cobble conglomerates in the uppermost portion of the MCF. All of our measured paleocurrents in the MCF are S-SW directed and indicate derivation from the north via the Beaver Dam Wash and/or from the northeast via the VRG. These results rule out a paleo-Colorado River that exited the Grand Canyon in the Grand Wash trough as the source of MCF strata. Conglomerate clast counts and sandstone petrography reveal that almost all clasts (>90%) north of the VRG are volcanic in origin and thus rule out a paleo-Colorado River source for strata to the north of the VRG. Detritus, as characterized in clasts counts, sandstone petrography, and detrital zircon dating, which are deposited south of the VRG are a mix of volcanic and other sediments that include ties to classic Colorado Plateau stratigraphic units. We analyzed quartz monzonite from the Henry Mountains laccolith and the Pine Valley units near St. George that currently drain into the Colorado and Virgin Rivers, respectively and are able to mineralogically distinguish them. Similar analyses on clasts in the MCF, from Sandy Point, and Callville Bay reveal Henry Mountain clasts at Sandy Point and Callville Bay but none were seen in the MCF. We conclude that the MCF could have been deposited by either a paleo-Virgin River or a paleo-Colorado River that once flowed through the VRG. Although our data do not refute the paleo-Colorado River hypothesis it seems more plausible that the MCF was deposited by a paleo-Virgin River.